A drug interaction is a situation in which a substance affects the movement of a drug, i.e. the effects are increased or decreased, or they create a new effect that neither produces on its own. Classically, interaction between drugs comes to mind (drug-drug interaction). However, interactions may also survive between drugs & foods (drug-food interactions), as well as drugs & herbs (drug-herb interactions).

Generally speaking, drug interactions are greatest avoided, due to the possibility of poor or surprising outcomes. However, in some instances, drug interactions have been intentionally used to great effect, such as the co-administration of probenecid with penicillin proceeding to mass production of penicillin. Because penicillin was difficult to manufacture, it was worthwhile to find a way to decrease the amount of penicillin required for a course of therapy. Since probenecid reduces the emission of penicillin from the body, a dose of penicillin will stay around the body for a longer period of time when taken with probenecid. Hence, probenecid allows individuals to take less penicillin over classes of therapy. In the present day, probenecid is usually no longer used for this purpose, since penicillin is now produced on a large-scale basis.

A contemporary example of a drug interaction used as a benefit is the co-administration of carbidopa with levodopa (obtainable as Carbidopa/levodopa). Levodopa is used in the management of Parkinson's disease and must attain the brain in an un-metabolized state to be beneficial. When given by itself, levodopa is metabolized in the peripheral tissues exterior the brain, which decreases the effectiveness of the drug and increases the risk of unfavorable effects. However, since carbidopa inhibits the peripheral metabolism of levodopa, the co-administration of carbidopa with levodopa allows extra levodopa to reach the brain un-metabolized and also reduces the risk of side effects.

Drug interactions may be the result of different processes. These processes may include alterations in the pharmacokinetics of the drug, such as alterations in the Absorption, Distribution, Metabolism, and Excretion (ADME) of a drug. Alternatively, drug interactions may be the effect of the pharmacodynamic properties of the drug, e.g. the co-administration of a receptor adversary and an agonist for the same receptor.